It's still there, sort of. Lookup "Garage Unlocker" in mods. the garage functions aren't there but the map still is. https://www.beamng.com/resources/garage-unlocker.9308/updates I just tried it and then took the screenshot. Map still works.
"Garage mode and access to the Garage level have been temporarily removed pending improvements to the way the Garage is integrated into BeamNG.drive gameplay" directly copied from the devblog
Some people prefered the UI of the garage over the parts selector. Personally I don't see why, but some people did
I believe the garage allowed you to select a different vehicle from your default one, before entering the game in the map (level) you want. It saved an extra step of first having to go into your map w/ your default veh, & then having to switch veh's there. Though I could be wrong? I'll admit I hardly used it myself... except for the situation I described above.
Yeah, that was it's main purpose, but when they removed it they allowed you to set a default vehicle straight from the vehicle selector. I prefer it this way, at least until they can implement the garage in a way that isn't overly redundant.